<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- This file documents the GNU Assembler "as".

Copyright (C) 1991-2016 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, with no Front-Cover Texts, and with no
Back-Cover Texts.  A copy of the license is included in the
section entitled "GNU Free Documentation License".
 -->
<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
<head>
<title>Using as: Comm</title>

<meta name="description" content="Using as: Comm">
<meta name="keywords" content="Using as: Comm">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<link href="index.html#Top" rel="start" title="Top">
<link href="AS-Index.html#AS-Index" rel="index" title="AS Index">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="Pseudo-Ops.html#Pseudo-Ops" rel="up" title="Pseudo Ops">
<link href="Data.html#Data" rel="next" title="Data">
<link href="CFI-directives.html#CFI-directives" rel="prev" title="CFI directives">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.smallquotation {font-size: smaller}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
div.indentedblock {margin-left: 3.2em}
div.lisp {margin-left: 3.2em}
div.smalldisplay {margin-left: 3.2em}
div.smallexample {margin-left: 3.2em}
div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
div.smalllisp {margin-left: 3.2em}
kbd {font-style:oblique}
pre.display {font-family: inherit}
pre.format {font-family: inherit}
pre.menu-comment {font-family: serif}
pre.menu-preformatted {font-family: serif}
pre.smalldisplay {font-family: inherit; font-size: smaller}
pre.smallexample {font-size: smaller}
pre.smallformat {font-family: inherit; font-size: smaller}
pre.smalllisp {font-size: smaller}
span.nocodebreak {white-space:nowrap}
span.nolinebreak {white-space:nowrap}
span.roman {font-family:serif; font-weight:normal}
span.sansserif {font-family:sans-serif; font-weight:normal}
ul.no-bullet {list-style: none}
-->
</style>


</head>

<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
<a name="Comm"></a>
<div class="header">
<p>
Next: <a href="Data.html#Data" accesskey="n" rel="next">Data</a>, Previous: <a href="CFI-directives.html#CFI-directives" accesskey="p" rel="prev">CFI directives</a>, Up: <a href="Pseudo-Ops.html#Pseudo-Ops" accesskey="u" rel="up">Pseudo Ops</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="g_t_002ecomm-symbol-_002c-length-"></a>
<h3 class="section">7.11 <code>.comm <var>symbol</var> , <var>length</var> </code></h3>

<a name="index-comm-directive"></a>
<a name="index-symbol_002c-common"></a>
<p><code>.comm</code> declares a common symbol named <var>symbol</var>.  When linking, a
common symbol in one object file may be merged with a defined or common symbol
of the same name in another object file.  If <code>ld</code> does not see a
definition for the symbol&ndash;just one or more common symbols&ndash;then it will
allocate <var>length</var> bytes of uninitialized memory.  <var>length</var> must be an
absolute expression.  If <code>ld</code> sees multiple common symbols with
the same name, and they do not all have the same size, it will allocate space
using the largest size.
</p>
<p>When using ELF or (as a GNU extension) PE, the <code>.comm</code> directive takes
an optional third argument.  This is the desired alignment of the symbol,
specified for ELF as a byte boundary (for example, an alignment of 16 means
that the least significant 4 bits of the address should be zero), and for PE
as a power of two (for example, an alignment of 5 means aligned to a 32-byte
boundary).  The alignment must be an absolute expression, and it must be a
power of two.  If <code>ld</code> allocates uninitialized memory for the
common symbol, it will use the alignment when placing the symbol.  If no
alignment is specified, <code>as</code> will set the alignment to the
largest power of two less than or equal to the size of the symbol, up to a
maximum of 16 on ELF, or the default section alignment of 4 on PE<a name="DOCF1" href="#FOOT1"><sup>1</sup></a>.
</p>
<p>The syntax for <code>.comm</code> differs slightly on the HPPA.  The syntax is
&lsquo;<samp><var>symbol</var> .comm, <var>length</var></samp>&rsquo;; <var>symbol</var> is optional.
</p>
<div class="footnote">
<hr>
<h4 class="footnotes-heading">Footnotes</h4>

<h3><a name="FOOT1" href="#DOCF1">(1)</a></h3>
<p>This
is not the same as the executable image file alignment controlled by <code>ld</code>&rsquo;s
&lsquo;<samp>--section-alignment</samp>&rsquo; option; image file sections in PE are aligned to
multiples of 4096, which is far too large an alignment for ordinary variables.
It is rather the default alignment for (non-debug) sections within object
(&lsquo;<samp>*.o</samp>&rsquo;) files, which are less strictly aligned.</p>
</div>
<hr>
<div class="header">
<p>
Next: <a href="Data.html#Data" accesskey="n" rel="next">Data</a>, Previous: <a href="CFI-directives.html#CFI-directives" accesskey="p" rel="prev">CFI directives</a>, Up: <a href="Pseudo-Ops.html#Pseudo-Ops" accesskey="u" rel="up">Pseudo Ops</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
</div>



</body>
</html>
